"No effort should be spared in determining the cause of this horrific incident," Martins said after visiting the accident scene on Monday afternoon.

"Road traffic management authorities must leave no stone unturned in their probe.

"There are no words to describe the shock with which we received the sad and unfortunate reports of this tragic end to lives."

On Monday morning, the bus driver lost control of the vehicle which hit the rails of a bridge, then plummeted 10 metres to the ground, landing on its side.

Beeld newspaper reported that the survivors blame the driver for the crash.

Elizabeth Xaba, 45, who is being treated in the Sebokeng Hospital, told the newspaper the driver did not slow down when he took the turn-off to the bridge.

"He was driving way too fast and he lost control. I only realised later that we had landed in the stormwater ditch. I could hear many people's voices... some of them were screaming in pain."

Passenger Ishmael Thomas, 35, who sustained at least eight broken ribs and a damaged lung, said he felt as if he was dying.

"The driver was speeding. Since the accident, I feel like I can't breathe."

Martha Nhlapo, 36, who was waiting for medical treatment in the Sebokeng Hospital casualty ward, said she remembered the driver hurtling along just before the crash.

"There was nothing in front of him that he might have had to avoid crashing into. No taxi or dog... he just drove too fast."

The Times quoted police inspector Albert Tsubella saying one of the survivors said the driver had been confused by passengers after taking a different route to avoid a service delivery protest.

The passengers, said Tsubella, started suggesting different alternative routes to the driver.

"The driver was either speeding or applied the brakes too late, or the brakes failed. This caused the bus to crash through a barrier into the ravine," said Tsubella.

Fifty-five people were injured in the crash -- three of them critically. The bus driver was among those killed.

"We call on the bus operating company to work with authorities in order to expedite the investigation into the cause of this crash," Martins said.

"Motorists must exercise great caution at all times. They should remember that adherence to the rules of the road is non-negotiable. It is only if we all obey the rules of the road without exception that we can meet our objectives of reducing the road fatality rate."

Martins conveyed his condolences to the families of the passengers who were killed, and wished the injured a speedy recovery.